Fix unit test timeouts on Travis CI

Jest recommends using 'runInBand' for Travis CI. It runs all tests
serially in the current process, rather than creating a worker pool of
child processes that run tests.

package.json

@@ -19,7 +19,7 @@
     "prerelease": "yarn test && yarn compile",
     "release": "node ./scripts/release/publish.js",
     "postrelease": "yarn website:release && yarn benchmarks:release",
-    "test": "yarn flow && yarn lint:check && yarn jest"
+    "test": "yarn flow && yarn lint:check && yarn jest --runInBand"
   },
   "devDependencies": {
     "babel-cli": "^6.26.0",